Most radios operate in multipath environments. In such multipath environments, more than one transmission path exists between the transmitter and receiver. This is because the transmitted signal can be reflected off of various interfering surfaces as it travels to the receiver.
Narrowband radios suffer in multipath environments due to the frequency selective nature of the phenomena. Narrowband radios can employ rake receiver structures to combine signals from the multiple paths, but this is a difficult and expensive process since narrowband systems lack the time-domain resolution to easily resolve the multipath terms. Rake is a term used to describe the coherent combining of energy from a plurality of multi-path induced replicas of the desired signal.
By definition, however, ultrawide bandwidth (UWB) systems have high time-domain resolution, and thus can resolve multipath signals. High chipping rate UWB systems have the advantage of operating in quasi-stationary multipath environments where the multipath is changing much slower than the code duration.
Each of the multiple paths in a multiple path system, whether direct or reflected, may well have a different length and so will cause the signal to arrive at a different time. A raking receiver may be used when multiple paths exist between two radios. FIG. 1 is a block diagram of a wireless system having two radios in which there are multiple transmission paths between the two radios.
As shown in FIG. 1, the wireless system 100 includes first and second radios 110 and 120, having first and second antennas 115, 125, respectively. There is a direct line of sight path 140 between the two radios 110 and 120, but there are also indirect paths 152, 154, 156 caused by bouncing signals off of other objects 132, 134, 136 in the area around the two radios 110 and 120.
As a result, if the first radio 110 sends a pulse out of the first antennae 115, the second antennae 125 will receive a plurality of pulses having an arbitrary spacing that correspond to that signal as it passes along one direct path signal 140 and multiple different reflected paths 152, 154, 156. And although FIG. 1 shows only three reflected signals 152, 154, 156 bouncing off of three objects 132, 134, 136, there can be many more reflections off of multiple other objects. In rooms you can have hundreds, even thousands, of reflections with all kinds of different reflected path lengths. In addition, although each of the reflected signals are shown as bouncing once off of a single interfering object 132, 134, 136, paths that have multiple bounces are also possible.
Furthermore, depending on the properties of each object 132, 134, 136, the strongest signal received at the second antenna 125 may be a reflected signal 152, 154, 156 rather than the direct signal 140. One reason for this is that there could be something collecting energy at one of the objects 132, 134, 136 and focusing it towards the receiving antennae 125.
Another reason that a reflected signal may stronger than a direct signal is that there could be multiple objects that cause reflections having the same reflected path length. For example, if a first reflected path 152 has a length L1, a second reflected path 154 has a length L2, and L1=L2, the two path lengths will be exactly matched. As a result of this, one pulse will travel from the first antennae 115 along the first reflected path 152 to the second antenna 125, and another pulse will travel from the first antenna 115 along the second reflected path 154 to the second antenna. But since the path lengths are the same, both pulses will arrive at the second antenna 125 at the same time and they would add their strengths together. Therefore it is not necessary that the shortest path signal be the strongest one received at the receiver.
FIGS. 2A–2C are graphs showing examples of the strengths of received signals in a multipath environment. In particular, FIGS. 2A–2C show the strengths of signals received at the second antenna 125 when a single pulse is output from the first antenna 115 and travels only along the three paths 140, 152, 154, and 156 of FIG. 1.
As shown in FIG. 2A, four pulses 205, 210, 215, and 220 arrive when the paths 140, 152, 154, and 156 are of different length and the signal strengths are about the same size. FIG. 2B shows four pulses 225, 230, 235, and 240 coming in, where the paths 140, 152, 154, and 156 are of different length but the signal strength of one path is much larger that the other paths. As a result, one of the pulses 240 is larger than the other three. FIG. 2C shows only three pulses 245, 250, and 255 being received because the pulses from the two reflection paths (e.g., first and second reflected paths 152 and 154) have the same path length (i.e., L1=L2). As a result, the two reflected pulses add their strength and so the third pulse 255 in this instance is larger than the first or second pulses 245 and 255 (i.e., the pulses from the direct path 140 and the third reflected path 156).
Thus, each path, whether direct (140) or reflected (152, 154, 156), may well have a different length and so will cause the signal to arrive at a different time. The receiving portion of each device 110, 120 must be capable of accounting for these different path lengths.